Here's Hopi asking why lefty columnists don't make the transition to Parliament in the way that right-wingers do*. I'd agree with all of his conclusions.
I'd add that the Tories seem to be able to tolerate people doing something that Labour has never managed. Right-wing journalists - in opposition, without any fixed positions to defend, have said the kind of things that Labour don't want to hear.
These are often nihilistic messages. Like The Housewives League of the 1940s, The Taxpayers Alliance (to give one example) has help to shift the debate rightwards. They've been able to do this, not by being coherent, but by helping to spread a damaging meme that necessarily reflects more badly on the government than the opposition.
And they're good at tarring Labour with the brush of profligacy and illiberality. Any columnist that took that semi-detached approach to Labour's policies would be decried as an apostate. And Labour aren't stupid in doing so either - it's here that you see the real political orientation of Grub Street. As Hopi says, Boris doesn't cause Cameron problems because it simply gets ignored.
Labour has never enjoyed that luxury.
*There is Martin Linton though - remember?